BOSTON — President Barack Obama told a memorial service for the Boston bombing victims that “we will find” whoever carried out the attack that killed three people as investigators search for two men seen on a video of the scene shortly before the blasts.

Obama said Americans would not be intimidated by the twin blasts, which also injured 176 people in a crowd of thousands at the finish line of the world-famous marathon on Monday.

“If they sought to intimidate us, to terrorize us, to shake us from those values … that define us as Americans, it should be pretty clear by now that they picked the wrong city to do it to. Not here in Boston,” Obama said at the memorial on Thursday.

While investigators have made no arrests yet, Obama said of the perpetrator or perpetrators of the attack, “We will find you and you will face justice.”

Homeland Security Secretary Janet Napolitano earlier on Thursday in Washington confirmed that the FBI was searching for people seen on a video taken near the finish line.

“There is some video that has raised the question of those that the FBI would like to speak with,” Napolitano said in congressional testimony on Thursday. “I wouldn’t characterize them as suspects under the technical term. But we do need the public’s help in locating these individuals.”

The Boston bombings put Americans on edge and security was tightened in major cities across the United States. Mail sent to Obama and federal officials that authorities believed contained the deadly poison ricin reminded Americans of anthrax mail attacks in the wake of the Sept. 11 attacks nearly 12 years ago.

The memorial service took place a day after the FBI arrested a Mississippi man in connection with the letters. The FBI said there was no indication of a connection between the ricin letters and the Boston bomb attacks.

In a separate incident on Wednesday, an explosion at a Texas fertilizer plant killed as many as 15 people. Authorities do not yet know what caused the explosion.

Obama was also due to meet families of victims of the bombing and first responders while in Boston, a White House spokesman told reporters aboard Air Force One.

Hundreds of people crowded outside the Cathedral of the Holy Cross in Boston’s South End, about a mile from the bombing site, where police officers stood outside their squad cars, listening to Obama over the radio.

“President Obama knows how important the city of Boston is to the nation and the world,” said 55-year-old John Snyder, who had joined the line before sunrise. “He is bringing his light to us for much-needed healing.”

Investigators believe the Boston bombs were fashioned out of pressure cookers and packed with shrapnel. Ten victims lost limbs, and emergency room doctors reported plucking nails and ball bearings from the wounded.

Police had considered making an appeal to the public for more information at a news conference on Wednesday, a U.S. government source said, but the FBI canceled it after a number of delays.

Boston police and FBI officials said on Thursday that they had not determined whether they would publicly release more details of the investigation.

The bombs in Boston killed an 8-year-old boy, Martin Richard; a 29-year-old woman, Krystle Campbell; and a Boston University graduate student and Chinese citizen, Lu Lingzi.

Before his visit, Obama declared a state of emergency in Massachusetts, a move that makes federal funding available to the state as it copes with the aftermath of the bombing.

The crowded scene along the race course in central Boston on Monday was recorded by surveillance cameras and media outlets, providing investigators with significant video footage of the area before and after the two blasts.

Based on the shards of metal, fabric, wires and a battery recovered at the scene, the focus turned to whoever may have placed homemade bombs in pressure cooker pots and taken them in heavy black nylon bags to the finish line of the race.

Tens of thousands of people turn out to watch and run in the marathon, which comes on a state holiday and is one of New England’s best-attended sporting events.

“This is Boston, a city with courage, compassion and strength that knows no bounds,” said Menino, who was rolled to the podium in a wheelchair but stood for his remarks despite breaking a leg over the weekend. “We love the brave ones who felt the blast and still raced through the smoke with ringing in his ears … to answer cries of those in need.”